Set hard-coded defaults for `list<string>` values to array()

Summary: These should default to array() so they're safe to `foreach` over.

Test Plan: Grepped for 'list<string>'.

Reviewers: codeblock, btrahan, starruler, vrana

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D4600